New wireless communications standards developed by the 3rd-Generation Partnership Project, such as the High Speed Packet Access (HSPA) and Long Term Evolution (LTE) standards, allow an unprecedented flexibility in the scheduling of transmission resources among mobile terminals served by a cellular base station. In these systems, the packet scheduler plays a crucial role in providing the appropriate quality of service (QoS) for a variety of different service classes while simultaneously maintaining high resource utilization. As a result, the design of efficient and QoS-aware scheduling algorithms has received much attention from a research perspective. One particularly promising class of QoS-aware schedulers, for example, makes use of frequency-selective and time-selective propagation channel characteristics by exploiting “opportunism.”
In an LTE context, where Orthogonal Frequency-Division Multiple Access (OFDMA) and closely-related Single-Carrier Frequency Division Multiple Access (SC-FDMA) techniques are used to facilitate fine-grained assignment of link resources to individual mobile terminals, a fundamental design assumption for a centralized packet scheduler approach is that there is a cell-wise centralized scheduling entity (e.g., located in the base station, known in LTE as the evolved Node B or “eNodeB”) that ensures that intra-cell collisions do not occur. That is, the centralized packet scheduler ensures that multiple mobile stations in the cell do not use the same time-frequency resources. Indeed, minimizing or even eliminating intra-cell interference is instrumental in ensuring high spectrum efficiency and providing high throughput.
From a system design and standardization perspective, significant efforts have been dedicated to developing the necessary control plane support to allow the central scheduling entity and the communicating mobile terminals to allocate time and frequency resources in such a manner that appropriate QoS is maintained and resources are well utilized. The design of the control protocols (which manage scheduling requests and grants) generally must address the issues of keeping the control plane traffic reasonably low, supporting a high granularity of resource sharing, preferably both in time and frequency, and ensuring low delay for managing scheduling requests and grants. In state-of-the-art schedulers, simultaneously meeting these three basic requirements is challenging.
An alternative to centralized scheduling approaches is to provide a distributed mechanism for controlling the access of competing mobile stations to the communication medium (that is, time, frequency, power, code and other resources). Distributed scheduling or distributed medium access control (MAC) mechanisms are well known and in use in several wireless and wired systems. One example is the classic ALOHA protocol. Such solutions are part of today's cellular networks, for example, in the so-called random access channel (RACH) and (uplink) common packet channels (CPCH), and have been studied for user data transmissions employing distributed MAC.
To minimize collisions, a base station employing one of these distributed mechanisms broadcasts an access probability with which a mobile terminal should transmit on the random access channel or the associated preamble sequence. For example, a persistency value is used in UMTS to determine whether or not a RACH transmission is initiated in a particular transmission time interval. The physical RACH resources may be divided between different service classes in order to provide different priorities of RACH usage. However, since random access is used to acquire initial access to the network, this approach does not take into account user-application-specific QoS requirements and does not provide any means to ensure the fulfillment and QoS differentiation of user plane connections. On the other hand, the control plane overhead is minimized in this approach, since per-terminal request and grant messages are not necessary.
Refined multiple access schemes have been proposed in the context of wireless communications systems. One example is the Packet Reservation Multiple Access (PRMA) protocol. This scheme is a refinement of pure random access solutions. In this method, after a successful random access, the UE (user equipment) retains dedicated or shared resources for a certain period of time. The drawback is that resources are wasted if not fully used during the time window.
Multi-user Multiple-Input Multiple-Output (MU-MIMO) systems create multiple data streams, separated in the spatial domain, using multiple transmit and receive antennas. The spatially separated multiple data streams may use the same time and frequency resource without causing interference to one another. Thus, MU-MIMO technology can be viewed as a means for avoiding time- and frequency domain (intra- or inter-cell) collisions.
Centralized packet scheduling and random access with minimum control plane support represent two extreme cases in the sense that the former eliminates intra-cell collisions, at the expense of control plane complexity, while the latter regulates collisions indirectly, via admission and load control mechanisms, while using a simplified control plane. Existing cellular systems employ both centralized schedulers and random access channels for different traffic types and different purposes. In these systems, the two approaches exist side by side, as two distinct medium access control (MAC) mechanisms. However, cellular networks that operate with high spectrum efficiency and provide service for a mixture of best effort and QoS enabled services typically employ centralized schedulers for user data traffic. Indeed, this is the case for the evolved Universal Terrestrial Radio Access (E-UTRA), the air interface for LTE systems, in which a rigorous control plane support ensures that intra-cell collisions are eliminated.
This approach creates several unresolved problems in E-UTRA. One problem is excessive control plane complexity and control plane traffic overhead—managing the scheduler requests and grants is a complex task and creates overhead in the carried total traffic through the radio interface. This is exacerbated in LTE systems because of the fine granularity, both in time and frequency, with which the resources may be scheduled. A second problem is increased user plane delay, which occurs because a mobile terminal must request a scheduling resource and await the grant from the centralized scheduling entity before it can access the channel for packet data transmission.
In short, improved medium access control mechanisms are needed to ensure appropriate levels of quality-of-service and ensure high resource utilization in systems where resources may be scheduled with fine granularity. Minimizing the control plane overhead, both in terms of control messages and induced delay, is also desirable.